1. Field of Invention
The invention relates in general to a measuring method, a lithographic apparatus, a substrate and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more alignment sensors by which positions of marks on a substrate must be measured accurately. The time taken for these measurements conflicts with the need to maximize throughput of the lithographic apparatus, otherwise device production will not be cost-effective. Various types of alignment sensors and marks are known, including for example the ones disclosed in patents U.S. Pat. No. 6,297,876 (Bornebroek), U.S. Pat. No. 6,961,116 (den Boef) and in published patent application US 2009195768 A (Bijnen et al). In each of these examples, the position measurement is captured by moving the substrate and the alignment sensor relative to one another, without the substrate and sensor having to be brought to a stop. The position of the mark relative to the (known) position of the alignment sensor is measured optically, by scanning the marks with an optical spot. (It does not matter whether the sensor moves while the substrate is stationary, or only the substrate moves, or both move.) The alignment sensors should scan the marks as quickly as possible for optimum throughput, but this need for Speed places limits on the accuracy of the position measurements that can be obtained. It is inherently more difficult to acquire a position accurately in a short time, I addition, a measurement acquired in a short time, even if the measurement itself is perfectly accurate, will be susceptible to disturbance by dynamic positioning errors (vibrations, servo errors etc.) in the positioning of the substrate and alignment sensor.